Garbage disposal itself fails in a number of different ways
1. If the main seal begins to leak, water will get down into the motor. The motor will get wet, short out and fail to start.
The motor may hum or it may be completely quiet when power is applied.
Feel the bottom of the disposer and if necessary, remove the cover of the wiring connection box. With the connection cover and the power off, stick your finger up inside the disposer and feel for any rust or moisture. A leaky main seal will frequently cause rust and water to collect on the bottom of the disposer. If moisture is found replace the unit.
2. The grinding plate may break loose from the shaft of the motor and start to wobble around. Sometimes it makes loud noises as it scrapes the outer wall.
3. Teeth may break off of the grinding plate unbalancing it.
4. The motor starting switch may fail.
5. Occasionally, the seam between the grinding chamber and the top of the motor will leak or the mounting flange will rot out and the disposer will start leaking up near the sink. Once a disposer is more than five years old, the grinding chamber begins to rot from the inside. It is very difficult to take out and reinstall an old disposer without having it leak. The components are all corroded and will be difficult to reseal.